1. Field of the Invention
This invention relates to a linear measuring system comprising an elongate housing tube, which contains a metal strip provided with a scale, and a scanning unit, which is also accommodated in the housing tube and movable along the housing tube to scan the scale. Said strip is fixed at its ends and is tensioned by means of a spring, which acts on one end of the strip. Means are provided for adjusting the initial stress of said spring.
2. Description of the Prior Art
In such a measuring system the scale may comprise a multi-track coded scale for absolute measuring systems or an incremental scale for incremental measuring systems. In the latter case, one or more tracks provided with reference marks may extend beside the incremental scale. The scanning unit is adapted to scan the scale by optoelectronic, inductive, magnetic or capacitive mathods. In all cases, analog electric signals are obtained, which are optionally converted to digital signals and are processed and used for an indication of a result of the measurement or in a control system for a machine. The elongate housing tube is usually secured to a machine tool or in an industrial robot and the scanning unit is connected to a movable part, the displacement of which is to be measured, or vice versa. The elongate housing tube is in most cases slotted at its bottom and the scanning unit is movable along said tube by means of a sword, which extends out of the tube between sealing lips lining the slot.
In the known measuring systems of the present kind, as disclosed, e.g., in U.S. Pat. No. 3,816,002, the elongate housing tube is closed at its ends by covers and the scale-carrying strip is connected at its ends to said covers. One end of the scale-carrying strip is fixedly gripped by means of a clamp bracket. The other end of the scale-carrying strip is held in a slot formed in a bolt, which is adjustable in the longitudinal direction of the carrier tube by a screw. A spring is also provided, which tends to move the belt in a direction to tension the metal strip. The initial stress of the spring can be adjusted by means of the screw. In some known designs, the screw is accommodated in the elongate housing tube. Other known designs are provided with one or more tensioning screws, which are operable from the outside of the housing tube.
In order to facilitate the manufacture of the elongate housing tube, said tubes are almost always made of light metal, particularly aluminum, which has a much higher coefficient of expansion than the material of the machine bed to which the tube is secured. The change of the length of the tube in dependence on temperature results in changes of the tension of the metal strip so that the change of the length of the strip is larger than the change of the length of the strip which is due to its coefficient of expansion, which is lower than that of aluminum. Inherently, a steel strip would have approximately the same coefficient of expansion than the steel of which the machine bed is constructed. The changes in tension which are due to the differential thermal expansion and particularly to the large thermal expansion of the housing tube may result in errors in the scanning operation and in measuring errors, particularly in relatively long measuring systems.
In a known design, the metal strip is embedded in a groove of the elongate carrier and is secured in that groove by means of an elastically yielding adhesive or by other elastically yielding means. That arrangement comprises also tensioning means, but said tensioning means can be effective only in part and often cannot be effective throughout the length of the strip, particularly when the strip is relatively long. Above all, the adherence of the strip to the retaining means has the result that the tensile forces vary throughout the length of the strip so that its elongation varies too. Owing to the retaining means, the strip will follow any deviation of the housing tube from a straight line. In the described design, the scanning unit consists in most cases of a wheeled carriage, which is guided on the strip, which is supported in the described manner. The scanning operation may result in a change of the distance between the reading part of the scanning unit and the scale provided on the strip transeversely to the plane of the strip and in a displacement of said reading part in the plane of the strip, particularly toward the side edges of the strip. Said changes may result in signal changes and measuring errors. In consecutive scanning operations said displacements may be different at the same point of the scale so that even correcting means which may be provided to compensate said errors by the introduction of stored correcting signals will be ineffective at least in part.